Wireless communications standards continue to evolve to meet growing demands for transmitting and receiving voice, data, and video signals. As an example, the cellular LTE-Advanced (LTE-A) standard has been created and expanded for worldwide access and global roaming in a variety of wireless services.1 To boost capacity, the maximum transmission bandwidth of LTE-A can be increased to 100 MHz by employing spectrum aggregation technology.2-4 But as transmission bandwidths increase, so, too do the challenges to provide cellular base-station power amplifiers (PAs) capable of the linearity, efficiency, and output power to support these expanding wireless standards. Efficiency is of particular concern, as it impacts not only performance but also system operating costs.

Several methods for improving PA efficiency have been investigated, such as the use of Doherty amplifier formats, linear amplification using nonlinear components (LINC), and envelope elimination and restoration (EER) approaches. Doherty amplifiers are attractive for their relative low cost, simple fabrication, and only moderate complexity. But improving the linearity of these amplifiers is also an important requirement for these systems. A number of newer technologies have been applied to improving PA linearity, including the use of RF predistortion, digital baseband predistortion, feed-forward techniques, and feedback techniques. Among these different approaches, digital baseband predistortion shows great promise for achieving good amplifier performance with low cost and flexible implementation.

Figure 1 shows a block diagram for a high-frequency PA with a digital baseband predistortion linearization scheme. A digital baseband predistortion module generates an in-phase/quadrature (I/Q) predistortion signal which is the exact inverse of the PA’s baseband I/Q response. The direct modulator converts the I/Q predistortion signal to an RF/microwave signal. The direct modulator is used to capture the output of the PA with the predistortion signal characteristics. A Doherty PA is used as part of the block diagram for its good wideband performance and high efficiency.

The modulator and demodulator in the block diagram can be replaced by an intermediate-frequency (IF) transceiver. An analog-to-digital converter (ADC) capable of digitizing the output signals from an LTE-A amplifier is difficult to realize at this frequency and bandwidth. As a result, a direct modulator and demodulator make more sense for the wideband linearization of an LTE-A PA. As the bandwidth of the PA and the system increase, the performance levels of the modulator and demodulator become more critical to performance of the amplifier and the system. To better understand the relationships of the direct modulator and demodulator on the linearization of the PA, the key I/Q parameters affecting the modulator and demodulator were simulated and analyzed in order to develop a novel linearization approach for an LTE PA with 100 MHz digital baseband bandwidth. By means of these simulations and experimental results, it was found that an LTE-A PA with 100-MHz bandwidth can achieve an adjacent-channel power ratio (ACPR) of -48 dBc.